In recent years, development has been advanced for ad-hoc networks in which a plurality of communication devices (referred to as node devices hereinafter) are connected to each other in an autonomous-decentralized manner. Respective devices in an ad-hoc network construct networks autonomously in accordance with communication environments.
Specifically, in an ad-hoc network, no access points are provided and each node device relays a data frame received from a node device that is adjacent (referred to as an adjacent node device hereinafter), to a different adjacent node device on the basis of the network topology and thereby the data frame is transmitted to a final destination device such as a gateway device (referred to as a GW device hereinafter).
In this situation, each node device selects a route on the basis of route cost CRoute for transmitting a data frame. In other words, when there are a plurality of routes connected to a final destination device, definition of route cost CRoute determines which of the routes is selected by each device.
As representative definition of route cost CRoute, there is a method in which a hop count is used as route cost CRoute and a method in which the integrated value of error rates of a link is used as route cost CRoute.
FIG. 1, which will be described later in detail, is referred to, and there are two routes from node device N7 to node device N1, i.e., route (N7, N6, N5, N1) that is via node devices N6 and N5 and route (N7, N4, N3, N2, N1) that is via node devices N4, N3 and N2.
When the hop count is used as route cost CRouter the route cost CRoute of route (N7, N6, N5, N1) is “3” and route cost CRoute of route (N7, N4, N3, N2, N1) is “4”. Accordingly, the route (N7, N6, N5, N1), which has the smaller hop count, is selected. However, when the error rate is high in the link between node devices N6 and N5, which are apart from each other by a long distance, there is a possibility that a data frame does not arrive at node device N1. In other words, using the hop count as route cost CRoute decreases the arrival rate.
When the integrated value of error rates of a link is used as route cost CRoute and a link between node devices Ni and Nj is expressed by “link Ni-Nj”, the route cost CRoute of route (N7, N6, N5, N1) is a value obtained by integrating error rates of link N7-N6, link N6-N5 and link N5-N1. Similarly, route cost CRoute of route (N7, N4, N3, N2, N1) is a value obtained by integrating error rates of link N7-N4, link N4-N3, link N3-N2 and link N2-N1.
It is assumed that link N7-N4 and link N7-N6 have the same error rate, link N2-N1 and link N5-N1 have the same error rate, and the integrated value of the error rates of links N4-N3 and N3-N2 is smaller than the error rate of link N6-N5. In other words, it is assumed that the following formulas are satisfied. (error rate of link N7-N4)=(error rate of link N7-N6) (error rate of link N2-N1)=(error rate of link N5-N1) (error rate of link N4-N3)+(error rate of link N3-N2)<(error rate of link N6-N5)
In such a case, route (N7, N4, N3, N2, N1), having the smaller integrated value of error rates, is selected. By using the integrated value of error rates as route cost CRouter, the arrival rate can be increased, however, it increases the hop count, deteriorating the frequency usage efficiency and decreasing the maximum accommodation number.
As described above, the two factors are in trade-off relationship.
As another definition of route cost CRoute, there is for example definition of route cost CRoute that is proposed in Patent Document 1. Route cost CRoute proposed in Patent Document 1 is obtained by calculating the inverse number of a value obtained by multiplying the forward reception rate by the backward reception rate of each link in a route, i.e., (1/{(forward reception rate)×(backward reception rate)}), as link cost CLink of each link and by obtaining the integrated value of these link costs CLink. Thereby, the method proposed in Patent Document 1 calculates route cost CRoute by measuring the forward and backward reception rates of each link.
When link N7-N4 and link N7-N6 have the same error rate, link N7-N4 and link N7-N6 have the same link cost CLink in a case when link cost CLink is calculated by using the method proposed in Patent Document 1. Similarly, when link N2-N1 and link N5-N1 have the same error rate, link N2-N1 and link N5-N1 have the same link cost CLink.
Accordingly, in this case, route (N7, N4, N3, N2, N1) is selected only when the following condition is met. (link cost CLink of link N4-N3)+(link cost CLink of link N3-N2)<(link cost CLink of link N6-N5)
Thereby, the method proposed in Patent Document 1 can suppress increase in a hop count while securing an arrival rate.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2012-105258